When you refine petroleum you end up with only so much gasoline or diesel. You can shift that equilibrium point, but end up wasting energy if you go to far. Right now the US refineries ship excess diesel to Europe and European refineries ship excess gasoline to the US. If we both attempt to go diesel heavy for the entire it simply won't work.
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In addition (and my info might be stale here) there is a lack of similar infrastructure (as gasoline has) for supplying diesel to the pumps.